First of all, welcome to the site! Second, this is pretty gosh darn good for a first team. I concur with Ursatic. Leftovers is a good item, but not for every pokemon. It works well for Ferrothorn, Jellicent, and Kyurem-B, and maybe Hydreigon, but there are better items for Keldeo, Thundurus and Hydreigon. On hydreigon, you may want to consider Life-Orb, which boosts the power of all attacks by 30%, and if you desire recovery, roost may be used. On thundurus, you may desire life orb as well, but other attack boosting items such as expert belt as suitable. You could also run a lum berry to protect it from status. I would also advise Grass Knot or Focus Blast over Foul Play, as more often than not Thundurus T has higher offensive stats that is opponents. Last but not least, your team overall lacks crucial speed, but worry not!, because Keldeo can serve you well as a scarfer. Choice Scarf boosts its speed by one and a halve but locks it into a single move, but it makes it a great revenge killer.